For a secondary battery such as a lithium secondary battery, positive electrode active material made of solid solution material such as Liy[M1(1-b)Mnb]O2 or Liy[M1(1-b)Mnb]O1.5+c (M1 is a metallic element) has been studied with an aim of achieving high capacity (See JP 2004-538610).
However, in a secondary battery using positive electrode active material made of the above solid solution material, when charge and discharge is repeated, it becomes difficult for lithium (Li) to return back to positive electrode active material due to diffusion control at the end of its discharge life, thus complete discharge cannot be performed. For this reason, even when a state of charge (SOC) is determined by measuring the actual battery voltage, it may greatly differ from the actual state of charge. Accordingly, there is a problem that the secondary battery cannot be controlled accurately.